The present invention relates to a protection circuit to be used for protecting a load. Such a load may consist of an electronic circuit such as circuitry placed on a printed circuit board, hereafter abbreviated with PCB, or of a DC/DC converter, or more generally DC powered devices.
Protection circuits are used throughout the electronics industry and are for instance described in European patent EP 0 848 472. Basically such protection circuits are intended to protect the circuitry of the load against inrush currents during hot insertion. Similarly, when detaching the supply terminals from the load, for instance during extraction of the board from the power feeding connector, the protection circuit has to protect the load against possible damage.
In general also a fuse is coupled in series with one of the input supply terminals of the protection circuit. The function of such a fuse is to additionally protect the protection circuit as well as the supplies against overcurrents, due to for instance a possible short circuit of the load itself. Furthermore the protection circuit itself often includes a current limitation circuit for limiting the current through this protection circuit, in case of overcurrents drawn by the load or even due to a short caused by the load. This provides yet an additional degree of protection. Such a current limitation circuit may sense the current flowing through the protection circuit and provide for a limitation when this current exceeds a predetermined threshold value.
However in practice it has turned out to be very difficult for finding a good balance between this predetermined threshold value, upon which detection the current limitation circuit becomes active and starts to effectively limit the current through the protection circuit, and the selection of the fuse by selecting its nominal value in function of the nominal current consumed by the load. Different considerations are to be taken into account for this selection:
1. The fuse rating should be above the maximum current consumed by the board during normal operation.
2. The threshold value of the in rush current limitation should be as low as possible during insertion of the board.
3. The threshold value of the current limiting circuit should be above the fuse rating, to make sure that the fuse blows in case of a short at the input side of the load, but the “I-t”-curve of the fuse should not be crossed while the tank capacitors at the input of the load such as a DC/DC converter are charged during hot insertion. If-the set value of the current limitation circuit is not 3 to 4 times above the fuse rating, it will take too long for the fuse to blow in case of a converter short or other excessively low impedance path, resulting in thermal overload of the current regulation component. The typical failure mode for the current regulation component is a short, resulting in a temporary short (until the fuse blows) of the battery, which may affect system operation in case of a single fault.
4. The large spread in battery voltages (38 . . . 72 V when compatible with the ETSI & ANSI markets), results in a large spread of charges to be handled during hot insertion.
Due to the combination of these problems, an overload condition at the load that results in the current through the protection circuit to exceed the threshold current by a relatively small amount will not cause the fuse to blow, in a safe time frame, which nevertheless creates a possible safety hazard.